Template value inference: closing the UFCS gap for template value parameters (or, "foo".format(bla) as a template instantiation)

[FeepingCreature]

On Monday, 5 August 2019 at 08:42:29 UTC, Olivier FAURE wrote:
> On Monday, 5 August 2019 at 06:49:32 UTC, FeepingCreature wrote:
>> Since D templates have value parameters, this suggests an 
>> analogous process of template value inference. How would this 
>> look?
>>
>> void format(string fmt, T...)(fmt, T args);
>
> I'd suggest
>
>     void format(enum Fmt: string, T...)(Fmt fmt, T args);
>

Seems extraneous. Enums are not types, fmt is not a variable. 
You'd basically be messing with the type system (introducing 
static single-value types, similar to the range propagation 
magic) for the sake of syntax. Besides, we already have a syntax 
for template value parameters.

> That said, does the use case of needing UFCS with a template 
> for a formatting string come up *that* often?

I mean, we're using format!"" exclusively, for typechecked 
safety. We actually have a regex that translates "foo".format() 
into format!"foo"() form. It would be nice to not have to do 
that, especially since ufcs form really is more readable. More 
than that, though, it is such a logical extension of how 
templates already work that I think it should work for that 
reason alone.